Other floors at Lumon Industries are known to exist, thanks to information revealed on the displays visible in the Security Office. They indicate the existence of a bank of at least four elevators ostensibly capable of reaching 16 total floors.
From top to bottom, the visibly named floors are:
None of these floors are seen or described in season one. Burt, however, at one point mentions “the executive wing upstairs,” which may be a reference to the 11th floor.
Six further floors appear to exist below the fourth, but their names are not legible. Presumably the Severed Floor and Main Lobby are among them, though it is unclear whether the Testing Floor, which appears to be accesible by a separate elevator, is as well.